






























^•^ "•' A^ <* '^'^ 4.C» 











3 V 




^ -^^^ ^c^yi-i^^^o ,/\.^;:;v ./>g';^'> ./\.'-- 



- ^^x^ 










^"-^^^ 






























"^^0^ 



«5°^ 











, <^^ . o » » 






^-^. 



^'''%. 






S°-^ -»i 











/v ^^s?/ ^% 










--.0- , "^^ '-^-\<^ %'*"^??^^>^ ^%^'^f?^^.^ ^^/-*^^V 



Lq 



v^ 



^^--^^ '.^^ 





f^^': '"'^bv^^ 



Q_ rf 







O^ ' *- o , o ■' 



0^ •i'j?^- "^.^ 













'^..^^ 



«7 V, 









.40. 



' ••--•' ^^, ..«'' .•^-. %„ ./ .iV^". \,<.* Z,^^'-, "^o^^./ 






,4.^ . 



^^•n^^ 






.^^ \ 











•b*^ 
















o V 





















v^ 











.-^^^.^ 



.^^ 












-of 




,-^°^ 














rv - » * o 




^-..^^'^ 



5-'' V^-'f.^*^ A 



^ 4" %> ' 





\3 'o . , * A 



rv^ . « • 











.50^ 







a5°^ - 



















IC 


8980 



Bureau of Mines Information Circular/1984 




A Review of Phosphatic Clay 
Dewatering Research 



By Walter E. Pittman, Jr., Jerry T. McLendon, 
and John W. Sweeney 




UNITED STATES DEPARTMENT OF THE INTERIOR 



■ ■■■II IIIWIIIM 



Information Circular 8980 



A Review of Phosphatic Clay 
Dewatering Research 



By Walter E. Pittman, Jr., Jerry T. McLendon, 
and John W. Sweeney 




UNITED STATES DEPARTMENT OF THE INTERIOR 
William P. Clark, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



This report is based upon work done under an agreement between the Florida Institute of Phosphate 
Research and the Bureau of Mines. 



Library of Congress Cataloging in Publication Data: 



1H 



^<^' 






Pittman, Walter E 

A review of phosphatic clay dewatering research. 

(Bureau of Mines information circular ; 8980) 

Bibliography: p. 26-29. 

Supt. of Docs, no.: I 28.27:89^0. 

1. Phosphate industry— Waste disposal. 2. Clay wastes— Dewater- 
ing. I. McLendon, Jerry T. II. Sweeney, John W. III. Title. IV. Se- 
ries: Information circular (United States. Bureau of Mines) ; 8980. 



'-i^95.U4 [TD899.P451 622s [622'. 364] 84-7598 



? 



.3 



ti. 



_^ CONTENTS 

Oo Page 

^ Abstract 1 

Introduction 2 

Acknowledgments 3 

Mining and benef Iclatlon procedures 3 

Early dewaterlng experiments 4 

Fundamental studies of phosphatic clay characteristics and flocculatlon 7 

Early dewaterlng techniques 9 

Freeze-thaw techniques 9 

Crust development 9 

Overburden pumping test 10 

Dewaterlng with moving screens 10 

^ Sand-clay sandwich process 11 

ANDCO process 11 

Sand wick 13 

Swift, Inc. , In-line sand-clay mixing 14 

Techniques under current evaluation 14 

Estech sand-clay mix using the Envlro-Clear thickener 14 

Dredge-mix process 16 

Dredge process 18 

Sand spray process 20 

Developing technology 21 

Gardlnler, Inc., process 21 

Bureau of Mines rotary trommel 23 

Summary 25 

References 26 

ILLUSTRATIONS 

1 . Typical phosphate mine 4 

2 . Slurry pit and hydraulic monitors 5 

3 . Flowsheet for phosphate benef Iclatlon 5 

4. Aerial view of typical settling ponds 6 

5 . Sandwich construction for sand-clay disposal process 12 

6. Envlro-Clear clarlfler-thlckener 15 

7. International Minerals and Chemical Corp. process for clay disposal 17 

8 . Sand spraying process for clay disposal 20 

9 . Clar If lux thickener 21 

10. Gardlnler, Inc., process for clay disposal 22 

1 1 . Bureau of Mines rotary screen dewaterlng system 24 



i 
I 

6o 








UNIT OF MEASURE ABBREVIATIONS 


USED IN THIS REPORT 


Btu/lb 


British thermal unit per 


pound 


lb/in2 


pound per inch squared 


cm 


centimeter 




lb/ton 


pound per ton 


ft 


foot 




m 


meter 


gal 


gallon 




M 


molar 


gal/h 


gallon per hour 




mL 


milliliter 


gal/min 


gallon per minute 




mol wt 


molecular weight 


h 


hour 




% 


percent 


in 


inch 




ton/h 


ton per hour 


kW'h/ton 


kilowatt hour per ton 




ton/yr 


ton per year 


lb 


pound 




yd3 


cubic yard 


lb/ft2 


pound per foot squared 




yr 


year 



A REVIEW OF PHOSPHATIC CLAY DEWATERING RESEARCH 

By Walter E. Pittman, Jr., ^ Jerry T, McLendon, and John W, Sweeney 



ABSTRACT 

This Bureau of Mines study surveys the current state of technology 
and the various research efforts that have been undertaken to dewater 
the dilute phosphatic clays generated in the production of phosphate 
rock. The research described includes early dewatering experiments, 
fundamental studies of clays and of flocculation, and minor dewatering 
techniques such as freeze-thaw methods, the ANDCO process, moving 
screens, sand wick, crust development, and overburden pumping. Some 
methods that are currently being evaluated are described. They include 
the Estech sand-clay mix process using the Enviro-Clear thickener, 
sand-clay sandwiching, dredge mix, dredge process, and the sand-spray 
process. Developing technology for dewatering phosphatic clays also is 
described, including the Bureau of Mines rotary trommel method and the 
Gardinier process. 

^Technical information specialist, Tuscaloosa Research Center, Bureau of Mines, 
Tuscaloosa, AL; professor of the history of science, Mississippi University for 
Women, Columbus, MS. 

^Mining engineer, Tuscaloosa Research Center. 

-•Supervisory mining engineer, Tuscaloosa Research Center. 



INTRODUCTION 



The Florida phosphate industry current- 
ly produces more than 80% of the total 
U.S. supply of phosphate rock. The pro- 
duction of this critical mineral is ac- 
companied by the generation of large 
quantities of dilute phosphatic clays. 
The disposal of these phosphatic clays 
has been a problem to the Florida phos- 
phate industry since the introduction of 
large-scale earth-moving equipment such 
as draglines in mining and the use of 
flotation processes in ore benef iciation 
in the industry in the 1920' s, which 
resulted in much greater recovery, in- 
creased efficiency, and lower costs. As 
the needs of the world's farmers for fer- 
tilizer increased, the Florida phosphate 
industry expanded to meet the demand. As 
the Florida phosphate industry grew, the 
problem of disposal of the phosphatic 
clays grew proportionately through the 
years and today represents probably one 
of the mining industry's largest waste- 
handling problems . The phosphate indus- 
try eventually found that the most prac- 
tical way to handle the clays was to 
store them in large impoundments where 
they were allowed to slowly dewater natu- 
rally. This method was chosen on prag- 
matic and economic grounds; it worked and 
at an acceptable cost. 

As the industry underwent its vast ex- 
pansion during the last 20 yr, the clay 
management problem grew in size and im- 
portance. At the same time, the industry, 
came under increasing attack from envi- 
ronmental interest groups and under in- 
creasing attention from local. State, and 
national regulatory agencies. The con- 
ventional clay settling areas or ponds 
presented several problems. They were 
aesthetically unpleasing, particularly in 
their early stages of development. Stor- 
age of the plastic phosphatic clays be- 
hind dams presented the ever-present (if 
actually extremely remote) possibility of 
a dam failure. The land used for clay 
storage areas meant land withdrawn from 
other uses and the loss of considerable 
acreage in a region where land values are 
increasing rapidly. Immense amounts of 
water were tied up in the clays that were 



desired for alternative uses, agricultur- 
al, residential, industrial, and even for 
further phosphate mining and processing. 

The phosphate industry, as well as the 
Bureau of Mines, private consultants, and 
university researchers, have undertaken 
extensive research and development pro- 
grams to seek a solution to the clay dis- 
posal problem. The industry has been mo- 
tivated both by a public spirit and by 
the economic imperative to reclaim expen- 
sive land and water for alternative uses. 
Further inducement was provided by the 
legal imperative to reclaim after the en- 
actment of the Florida Severance Tax Act 
of 1972. 

Until 1972, the industry research ef- 
fort was largely the product of the work 
of individual phosphate companies , and 
the information was usually proprietary. 
In 1972, through the combined efforts of 
the phosphate industry and the Bureau, 
the Florida Phosphatic Clays Research 
Project (FPCRP) was established. The 
FPCRP was organized to coordinate the 
phosphate industry's activities, under 
the direction of L. G. Bromwell, working 
in concert with the Bureau. University 
researchers, private consultants, and 
other governmental agencies were also in- 
volved. In 1979, the Florida Institute 
of Phosphate Research (FIPR) , a State 
agency funded through a portion of the 
severance tax on phosphates , was formed 
and became an important center of re- 
search on phosphatic clays. 

Through the years , numerous techniques 
to dewater phosphatic clays have been de- 
veloped, which varied greatly in their 
efficiency and feasibility. Technical 
practicality has proven to be the biggest 
obstacle to developing effective dewater- 
ing techniques. In the last decade, a 
growing awareness of environmental issues 
has caused an increased interest in the 
phosphatic clay problem and has led to 
the involvement of environmental and reg- 
ulatory groups from outside the industry. 
All of these factors have increased 
the pressure upon researchers for an 



economic, simple, quick, and environmen- 
tally safe dewatering technique. 

It would seem that every conceivable 
dewatering technique has been suggested 
and that most of them have been tried at 
one time or another. Those that met the 
test for technical feasibility were then 
usually tested on a larger scale. During 
the same time period, parallel studies of 



a more fundamental nature 
ried out on the clays. 



have been car- 



This report is the result of a detailed 
review of phosphatic clay dewatering re- 
search by the FIPR, under Bureau memoran- 
dum of agreement 14-09-0070-954, and de- 
scribes research conducted by industry, 
academia, and government. 



ACKNOWLEDGMENTS 



The authors wish to express their ap- 
preciation to the Florida Phosphate Coun- 
cil, the Florida Institute of Phosphate 
Research (FIPR) , and the library staff at 
FIPR for making available records and 



unpublished data. Bromwell Engineering 
and Zellars-Williams Co. assistance in 
collecting and interpreting data is also 
acknowledged . 



MINING AND BENEFICIATION PROCEDURES 



Strip mining techniques are in general 
use in the Florida phosphate industry, as 
shown in figure 1. Large draglines are 
used to dig a series of parallel cuts, 
which are each several hundred to several 
thousand feet in length and 200 to 300 ft 
wide. The overburden is cast into a 
previously mined parallel cut, and the 
phosphate-bearing zone (matrix) is ex- 
posed. The matrix is mined by the drag- 
line and placed in a slurry pit, located 
above ground level and in reach of 
the dragline. In the slurry pit, large, 
high-pressure water guns (10,000 to 
12,000 gal/min at 200 Ib/in^) break down 
the matrix to produce a slurry (fig. 2), 
which is then pumped to the benef iciation 
plant. 

The method of phosphate benef iciation 
depends upon the type (size) of material 
that constitutes the slurrified feed. 
Phosphate pebble (fig. 3) is separated by 
a series of screens and log washers, or 
their equivalent, in a closed circuit. 
The pebble-size concentrate is utilized 
directly, while the finer material (minus 
35 plus 150 mesh) is conditioned and flo- 
tation reagents are added. The clays and 
sands are separated from the final phos- 
phate concentrate as waste tailings. 

Large quantities of clay waste are pro- 
duced in the benef iciation process (fig. 



3). A typical company, producing 3 mil- 
lion ton/yr of phosphate product , mines 
about 400 acres of land, removing 13 mil- 
lion yd^ of overburden, and produces 9 
million yd^ of matrix. For each ton of 
phosphate produced, approximately 1 ton 
of sand tailings and 1 ton of clays must 
be disposed of (J^).^ These figures vary 
widely, however, depending upon the na- 
ture of the phosphate matrix. The clays 
are dispersed in water as a dilute col- 
loidal suspension, and a typical mine can 
generate many millions of tons of phos- 
phatic clay tailings annually. Each 
phase of the mining and beneficiation 
operation contributes to the generation 
of phosphatic clays. Collectively, phos- 
phatic clay is composed of minus 150-mesh 
particles of clay, quartz, and phosphatic 
materials that are rejected during ben- 
eficiation. Generally, phosphatic clay 
slurries containing 3% to 5% solids are 
impounded behind dikes , which are built 
around mined-out pit areas. Approximate- 
ly 60% of this material is stored below 
ground and 40% above ground level. These 
dikes often range in height from 20 to 60 
ft above natural ground level and occupy 
as much as 300 to 800 acres each. Fig- 
ure 4 shows an aerial view of a typical 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 




FIGURE 1. - Typical phosphate mine. 

settling pond. It is estimated that each, area are needed for phosphatlc clay 
year 2,500 acres of additional storage disposal (2^). 

EARLY DEWATERING EXPERIMENTS 



Many novel technologies have been stud- 
ied in an effort to solve the phosphatic 
clay disposal problem. Among these have 
been numerous electrical methods. 

Studies of the electrophoretic charac- 
teristics of phosphatic clays have been 
undertaken by several private investi- 
gators as well as by the Bureau 0-5). 
Electro-osmosis and electrophoresis meth- 
ods have been investigated for their pos- 
sible use on phosphate clays. The tech- 
niques have been applied for some time to 



fine particle slurries, particularly as 
an aid to filtration or to dewater fine 
slurries underground (^~8^) • Attempts to 
use electro-osmotic techniques to dewater 
phosphate clays date back over 20 yr. 
While the technology "would appear to 
have promise," it is "not considered com- 
mercially feasible" (6^) except under spe- 
cial circumstances because of the slow 
dewatering rate of the clay wastes. Sig- 
nificantly increasing the dewatering rate 
is possible but involves prohibitive en- 
ergy costs. Bureau investigators have 




FIGURE 2. - Slurry pit and hydraulic monitors. 



From mine pit 



Trommel screens 



Hammer mill 
To waste 



V4 in J J 



I Minus 14 mesh 



Primory vibrating 1 Minus 14 mesh 
screens 



Primory log 
washers 



^ Minus 14 mesh 



-Minusl4_mesh 



j Secondary vibrating I 
screens 



Secondary log 
woshers 



Finishing screens 



Pebble product 
V4 in by 14 mesh 



WASHING PHOSPHATE ORE 



Feed sizing 



1 Intermediate pebble 
1 14 by 35 mesh 



YTZ ^ ^'"6 feed, _^ 

I " n 35 by 150 mesh P ] 

iblel I J Coarse feed. I J 
i__J "20 by 35 mesh[ ^ 



To conditioning 
and flotation— 



—►Anionic reagents 



Drum conditioner 



Spirols I ►Spiral concentrate to product storage 



™^9^^^f^9^M— ►Scavenger concentrate to deoiling 

1 1 flfiri nmin(> fintntinn 



and amine flotation 



Tailings 



INTERMEDIATE PEBBLE BENEFICIATION 



Intermediote 
pebble 

beneficiotion 
circuit 



Primary desliming [ 



Minus 



150 mesh 
Minus 



ISecondary deslimingi 150'!},"^^^ 



Feed sizing I 



♦To cloy 
disposal 



Dewatering | 



De watering 



I Condilioning | 



Anionic reogents 



I Conditioning | 



CoQrse(plus 35 mesh) Fine (minus 35 mesh) 



Rougher flotation 



Iki. 



Rougher toils 



I Rougher concsntrote I 
1 acid scrub and rinse 



Cotionic reogents 



Amine flotation j— 



-►To 
tailings 
disposal 



Clarification 
and refuse 



Fmol concentrate 
14 by 150 mesh 



FEED PREPARATION FLOTATION 



FIGURE 3. - Flowsheet for phosphate beneficiotion. 




FIGURE 4. - Aerial view of typical settling ponds. 



determined the electro-osmotic character- 
istics of fine phosphate clay wastes and 
were able to dewater clay wastes up to 
35% solids on a small (4— ton) scale. The 
power required to increase the solids 
content of the phosphatic clays from 17% 
to 25% was 25 kW*h/ton of water removed. 
The (DC) voltages used in the tests 
ranged from 8.8 to 245.5. As the clays 
thickened, higher voltages were required 
and power requirements also increased. 
There was also difficulty in removing the 
supernatant liquid from the clay wastes 
as the clay thickened and the migration* 
of the water became more difficult (9^) . 

Electrostatic benef iciation of phos- 
phate ore was investigated by Interna- 
tional Minerals and Chemical Corp. (IMC) 
in the mid-1950' s with a view toward eli- 
minating the need for the flotation pro- 
cess (10) . Investigators at Princeton 
University Plastics Laboratory investi- 
gated the potential use of nonuniform 
electrical fields for the continuous sep- 
aration of suspensions in a dielectric 
fluid, water (11) . The Tennessee Valley 
Authority (TVA) built and tested a rotat- 
ing anode machine, and the Bureau built 
and tested a continuous electrical dewa- 
tering system using a moving metal belt 



as the anode. These reduced the mois- 
ture content of the clay suspensions but 
failed to yield clear effluent water (10- 
15 ) . Magnetic separation techniques were 
applied to phosphate ores by a Massachu- 
setts Institute of Technology (MIT) group 
in 1972 and 1973, with a view toward 
avoiding the creation of phosphatic clay 
wastes. The process did not prove feasi- 
ble (16). Scientists at TVA, Virginia 
Polytechnic Institute, and IMC investi- 
gated the possible use of ultrasonic en- 
ergy to precipitate phosphatic clays as 
early as the 1950' s. However, a commer- 
cially feasible system was not developed 
at the time. In recent years, there has 
been a renewal of interest in the possi- 
ble use of ultrasonic energy to precipi- 
tate clay slurries ( 13 , 17 ) . 

An attempt was made in the early 1950 's 
to use cull citrus fruit , which repre- 
sented a large volume of waste, to mix 
and react with phosphatic clays and to 
solve a mutual disposal problem. The 
volume of clays produced by phosphate 
mines is so large that the process proved 
impractical without even considering oth- 
er difficulties such as transportation 
(18). 



The use of continuous centrifugation 
processes to dewater phosphatic clays was 
investigated by TVA and others. The use 
of centrifuges was determined to be im- 
practical, however, because of high capi- 
tal investment and energy requirements 
(12). 



Benef iciation of phosphate ore by using 
dry methods was investigated by a private 
company as a possible means of eliminat- 
ing the production of dilute phosphatic 
clays as a byproduct. The process devel- 
oped was not adopted because it proved 
impractical (19-20). 



FUNDAMENTAL STUDIES OF PHOSPHATIC CLAY CHARACTERISTICS AND FLOCCULATION 



I 



Through the years, numerous studies 
have been made of the mineralogical, 
physical, and chemical characteristics of 
phosphatic clays. These studies have 
largely been undertaken by industry it- 
self, by the Bureau, or by private or 
university consultants sponsored by the 
industry, the Bureau, FPCRP, or FIPR. 

Phosphatic clay samples taken from set- 
tling ponds were characterized by Bureau- 
sponsored researchers at Florida State 
University using an electron microscope. 
The clays were examined to determine the 
shape and texture of the clay particles 
and how these affected their flocculation 
and settling in disposal ponds. This was 
basic research aimed at understanding the 
fundamentals of clay particle behavior 
and is typical of numerous characteriza- 
tion studies of phosphatic clays un- 
dertaken by various investigators (21) . 
Other characterization studies also de- 
scribed the variations among the minerals 
making up the phosphatic clays throughout 
the phosphate mining district (22) . 

C. C. Ladd of MIT undertook basic con- 
solidometer and permeability tests on 
representative samples of Florida phos- 
phatic clays. Tests were also made in a 
test pit using various combinations of 
drains, consolidation rates, and pore 
pressure measurements , and to determine 
the effect of hydraulic gradients (23) . 

Screening of possible flocculants for 
use in dewatering phosphatic clays has 
been undertaken at various times by pri- 
vate companies , consultants , and the Bu- 
reau. Flocculation is a technique in 
which discrete, colloidal-sized particles 
are agglomerated by an appropriate rea- 
gent and, as a result, settle out of 



suspension (24) . Hundreds of commercial 
flocculating reagents have been tested 
singly or in combination with others, in 
an effort to select a flocculant that 
will result in the formation of stable 
floes that will not reslurry readily and 
that will cause rapid settling and dewa- 
tering of phosphatic clays. Frequently, 
successful flocculating reagents evalu- 
ated in the laboratory on a specific clay 
proved unpredictable in field tests owing 
to the variables encountered in the field 
test conditions. Among these variables 
are clay mineralogy, age of the clay 
slurries, the method of flocculant intro- 
duction, the dilution of the clay slur- 
ries, the pH of the slurry, the mixing 
shear, and the conditioning and contact 
time. A systematic evaluation of the 
many hundred available commercial floc- 
culants was undertaken by the Surface 
Chemists of Florida (SCF), in research 
sponsored by the FPCRP. SCF found that 
high-molecular-weight organic polyacryl- 
amide polymers were the most efficient 
but that galactomannans (guar) were about 
as good. It was found that while high- 
molecular-weight polymers were better 
flocculants . a point of diminishing re- 
turn was soon reached with increasing 
molecular weight. Anionic flocculants 
proved superior to neutral or cationic 
ones, and combinations of polymers were 
often more successful than single ones. 
In fact, combinations of poor flocculants 
together were extremely effective in some 
cases (25) . 

Several processes using chelating 
agents to hasten the settling of phos- 
phatic clays have been investigated, and 
some have been patented. None appear to 
have been adopted on a practical scale 
(26). 



Basic studies of the mechanism of floc- 
culation of phosphatic clays were under- 
taken by researchers at Auburn University 
for the Bureau. The effect of the com- 
pressive stress exerted by a column of 
clay wastes was first studied, and then 
small-scale models of thickening systems 
(fluid bed and the Lamella thickener) 
were built and tested. The conclusion 
tentatively drawn from the tests was that 
several thickeners used in series would 
result in effective and efficient dewa- 
tering of phosphatic clays (27-28) . 

A group of researchers at MIT, led by 
C. C. Ladd, in work sponsored by the 
FPCRP, found that phosphatic clays solid- 
ified in four stages or phases. These 
all occurred simultaneously but differed 
in degree of importance during the time 
progression of settlement. The first was 
sedimentation, in which the clays settled 
out of the supernatant water. Then came 
consolidation, in which the clays solidi- 
fied under their own weight, squeezing 
out entrapped and interstitial water. 
Consolidation generally followed Terza- 
ghi's theoretical model. Next came the 
stage of consolidation under hydraulic 
gradients. If the pore pressures at the 
bottom of a column (or depth) of consoli- 
dating phosphatic clays were reduced be- 
low hydrostatic levels, seepage would oc- 
cur as more supernatant liquid migrated 
down the column to equalize the pressure. 
This led to a formal recognition of a 
fact already known in practice: that 
bottom drainage from a phosphatic clay , 
holding area above natural ground greatly 
enhanced the dewatering of the clay. The 
last stage was that of surface stabiliza- 
tion. The presence of surface water 
tended to keep the surface weak. Removal 
of this water by promoting surface drain- 
age led to greatly enhanced drying and to 
the creation of a surface crust suffi- 
cient to bear weight. The most important 
idea brought out by the MIT-based re- 
searchers was probably the critical role 
of a column of weight pushing down and 
enhancing the consolidation of the phos- 
phatic clays (23) . 

Colloidal gas aphrons (CGA) are dis- 
persions of micrometer-sized bubbles that 



can concentrate the phosphatic clays in 
the dilute waste stream by a combination 
of flocculation and flotation. This pro- 
cess was evaluated by researchers at Vir- 
ginia Polytechnic Institute under a grant 
from FIPR. The technique of using CGA to 
minimize the water content of the clays 
and to separate phosphate values from the 
clays is being investigated, and various 
flocculants are being evaluated for cost 
and effectiveness (29) . 

FIPR also funded Zellars-Williams , Co., 
to study the effect that the addition of 
hydrated lime had upon phosphatic clay 
dewatering. It has been known that the 
addition of lime enhances water recovery 
and improves the material strength of 
the remaining solidified material. This 
study aimed at quantifying the effect of 
lime addition to dilute phosphatic clay 
and trying to determine the final amount 
of stabilization possible in dewatered 
clays. The effect of residual organic 
flocculants and the addition of sand 
tailings or phosphogypsum to the lime- 
treated clays were additional parameters 
that were studied ( 30 ) . 

IMC, with FIPR support, is testing a 
method of disposing of phosphatic clays 
in old mine cuts. The partially dewa- 
tered clays are pumped into the mine 
cuts. The overburden windrows give good 
access to the disposal area and make less 
difficult the task of placing a sand or 
soil cap on the clays after they have 
dewatered further. The mine cuts also 
present a greater surface area for dewa- 
tering drainage than conventional dispo- 
sal ponds. A full-size test pit was 
filled and evaluated (31) . 

Basic chemical, mineralogical, and me- 
chanical characteristics data were col- 
lected by Bromwell Engineering for an 
FIPR project. Phosphatic clay samples 
were taken from disposal areas, current 
mining operations, and locations desig- 
nated for future mining. The tests in- 
cluded chemical analyses (P, U, Ra, Al, 
Fe, Mg, F, Ca, and cation exchange capac- 
ity) , mineralogical analyses (X-ray dif- 
fraction and scanning electron micro- 
scopy) , and physical analyses (grain-size 



distribution, plasticity indexes, viscos- 
ity, settling characteristics, and slurry 
consolidation behavior). These data, 
when analyzed and published, will provide 
a foundation of clay characteristics for 
future research (32) . 

Research was undertaken by the Bureau 
and by private and university investiga- 
tors under Bureau sponsorship to see if 
micro-organisms could be used to promote 
aggregation of clay particles from dilute 
phosphatic clay wastes. Some fungi were 
found to be effective but only when their 



growth had been greatly enhanced by add- 
ing nutrients to the clay slurry. This 
was judged to be an impractical process 
at the present time (33) . 

Numerous investigations have been made 
by the Bureau, State laboratories, uni- 
versity scientists, and private companies 
of the use of ion-exchange techniques in 
dewatering phosphatic clays. In numer- 
ous similar industrial processes, ion- 
exchange technology has proven success- 
ful. However, to date no ion-exchange 
process has proven practical (34-37). 



EARLY DEWATERING TECHNIQUES 



Several dewatering techniques exhibited 
enough promise at the laboratory scale 
that large-scale tests were carried out 
in the field. 

FREEZE-THAW TECHNIQUES 

Freeze-thaw techniques have been used 
successfully on a small scale to dewater 
fine clay wastes and sludges. When the 
wastes are frozen, the water separates 
from the clay particles and freezes as 
ice. Upon thawing, the clay particles 
remain dehydrated and settle as a rela- 
tively high-density concentrate from 
which most of the water can be decanted. 
Bureau investigators found that phos- 
phate clays averaging 13.7% solids can be 
quickly frozen and thawed to produce a 
settled clay fraction comprised of 42% 
solids. The energy costs were calculated 
at 183 Btu/lb of clay wastes. At that 
time, industry found these energy re- 
quirements to be "prohibitively high," 
and the technology to treat the large 
areas required has not been developed 
(38-40) . 

NTP Corp., under a grant from FIPR, is 
currently testing a thermal process for 
the rapid dewatering of phosphatic clays. 
The system is a freeze-melt one using 
n-butane in a vapor compression refriger- 
ation cycle to f reeze-separate the col- 
loidal clay-water suspension. Prethick- 
ened phosphatic clays (18% solids) are 



pumped through a precooling heat ex- 
changer and into a freeze tank where liq- 
uid n-butane is bubbled through the clay 
slurry. The expanding n-butane cools the 
slurry, and ice forms and is removed. 
The n-butane is recovered and recycled. 
Two methods for removing the thickened 
phosphatic clays are under study: In 
one, the clays move with the ice slush 
and are separated in the melting water 
cycle; in the other, the thickened clay 
solids are removed from the bottom of the 
freeze chamber (41) . 

CRUST DEVELOPMENT 

Several approaches have been pursued 
with the aim of producing a crust upon 
waste clay settling ponds capable of 
bearing a substantial weight. If suffi- 
ciently hardened, in a reasonable time, 
the crust can then support equipment cap- 
able of spreading other, more permanent 
surface materials such as sand tailings 
and overburden. One approach that has 
been successful is to allow a vegetative 
cover to appear. This is usually com- 
prised primarily of cattails. The vege- 
tative surface offers much greater 
bearing capacity than the clay surface 
although it is relatively slow to become 
established. Encouraged by these re- 
sults, IMC systematically sought out and 
tested various grains and grasses for 
their growth potential as well as for 
their shear strength properties. IMC 



10 



found that Japanese millet thrived in the 
phosphorous-rich clays, was able to tol- 
erate the watery environment, and pro- 
duced a bearing surface of great strength 
in a short time. In 10 weeks, the Japa- 
nese millet grew to a height of 2 ft and 
had an average root depth of 1-1/2 ft. 
Furthermore, it naturally reseeded itself 
and displaced the other test species in 
the pit. 

Efforts to aid the desiccation of the 
clay settling area surface itself were 
also undertaken. Mobil Chemical Co. at- 
tempted to apply a method that had been 
successful elsewhere in dewatering fine 
particle sludge, the Hardaman crust 
disruption system. The fine suspensions 
(clay wastes in this case) are pumped 
into wide, shallow holding areas where 
they are allowed to settle. The clear 
water is drawn off, and the remaining 
solids dry in the air and sun. When a 
crust has formed, tractors are used to 
break up the surface and expose new mate- 
rial to drying. However, this method was 
found not to work with Florida phosphate 
clays because of their slow settling 
rate, the high ambient humidity, the 
heavy rainfall, and the vast areas of 
land that would be required. 

Currently, efforts are underway to de- 
velop specialized equipment for use on a 
partially solidified clay settling pond 
surface. One problem in reclamation of 
mined-out areas has been the long time 
periods needed to develop a surface crust < 
sturdy enough to support equipment that 
can cap the wastes and thereby hasten the 
return of the mined-out area to other 
usage. Special high-flotation vehicles 
are being developed, and at least one 
company is experimenting successfully 
with the use of such equipment to spread 
sand tailings over the semisolid surface 
of a clay waste settling pond (42-43) . 

OVERBURDEN PUMPING TEST 

In 1972, Mobil Chemical Co. undertook 
tests using slurrified overburden to cap 
clay wastes. Based on preliminary labo- 
ratory data, an 80-acre test site was es- 
tablished in a mined-out pit. Overburden 



from an active mining site was slurrified 
and pumped to the test site where it was 
mixed with the phosphatic clays from the 
phosphate processing plant. The tests 
were not successful. There were several 
reasons for the failure. The overburden 
included quantities of clay, which, when 
slurrified, produced even more fine sus- 
pended clay particles and made the prob- 
lem of disposal worse. The overburden 
and phosphatic clays tended to separate 
and the clay to migrate. The overburden 
entrapped very little clay. The slurri- 
fication process also put into water sus- 
pension organic materials, which formed 
very stable suspensions. The resulting 
water turbidity required a separate clar- 
ification process (44) . 

DEWATERING WITH MOVING SCREENS 

Bureau researchers investigated the use 
of moving screens as a potential method 
of dewatering phosphatic clay wastes. 
The method was based on the idea that if 
the gel-like structure of the colloidal 
clay wastes suspension were gently broken 
up mechanically, it would free a large 
percentage of the interstitial water at 
an acceptable cost. 

Laboratory tests were performed on 18 
industry-supplied phosphatic clay waste 
samples . These samples , from different 
plants, varied in composition and ranged 
from 2.6% to 16.7% solids content. Small 
screens (4, 8, or 16 mesh) were slowly 
moved through the 250-mL columns of clay 
wastes, and the supernatant water was 
periodically removed. Similar tests were 
carried out on 8-gal samples using the 
same screen sizes (4, 8, or 16 mesh) but 
arranged so that there were three screens 
(of the same size) mounted one above the 
other on a shaft. 

The moving screens effectuated signifi- 
cant dewatering in a relatively short 
time period. Typical samples, with 4.7% 
and 11.9% solids content, dewatered to 
16.8% and 25.2% solids content, respec- 
tively, in 3-1/2 days. An attapulgite 
sample (2.7% solids content) dewatered to 
13% solids in the same time period. 



11 



One interesting effect, of possible in- 
dustrial interest, was that dewatering 
extended significantly beyond the volume 
through which the screens moved. Screen 
size was found to have no significant ef- 
fect; the 4-, 8-, and 16-mesh screens 
produced roughly equivalent dewatering. 
The speed at which the screens were moved 
proved, however, to be a critical vari- 
able. To achieve significant dewatering, 
the screens had to move with extreme 
slowness. Significantly, it was seen 
that clay wastes that had naturally poor 
settling characteristics, such as atta- 
pulgite, produced higher percent solids 
as a final product by moving screen com- 
paction than those that settled more rap- 
idly under normal conditions. It was 
also found that the most dilute samples 
(lowest percent solids content) lent 
themselves to the greatest degree of de- 
watering by the moving screens. 

The process appears to be too slow, 
however, to cope with a large flow of 
clay wastes but might be suitable for 
special usage with particularly difficult 
clays ( 45 ) . 

SAND-CLAY SANDWICH PROCESS 

The difficulties of mixing sand and 
clay led to experiments in which alter- 
nate layers of sand and clay were put 
down, with the sand tailings layers pro- 
viding drainage paths and putting weight 
on the phosphatic clays below, which aid 
dewatering. 

A test site was created at USS Agri- 
Chemicals Rockland Mine. The test pit 
was 40 by 5 by 7 ft deep. It was found 
that it took about 30 days for the 3.5% 
solids content of phosphatic clays to 
reach a consistency that would support a 
gently emplaced sand tailings layer (17% 
to 23%). Phosphatic clays (3.5% solids) 
were added to the 7-ft test pit in layers 
4 to 6 ft thick. Within 30 days these 
had naturally dewatered to such an extent 
that they remained only 7 to 19 in thick. 
Sand tailings were then gently added in 
thin layers. The experimenters were 
careful to connect the sand layers to the 
sand dam at the outlet end of the pond so 



as to provide an adequate drainage path 
(fig. 5). An equivalent total depth of 
42 ft of clays averaging 3.5% solids con- 
tent were added, through a 308-day peri- 
od, to the 7-ft pit. The final thickness 
of the clay bodies was 42 in, and the 
clay solids averaged 33.8%. The design 
of the experiment was aimed at creating a 
400- to 600-lb/ft2 pressure on the clay 
wastes. This should give a final clay 
solids content exceeding 35%, which occu- 
pies a small enough volume that all the 
benef iciation waste products can be 
stored below grade level. The weight is 
provided by the alternate sand and clay 
layers pressing on those below. Later 
tests were carried out using a floccu- 
lated clay-sand mixture (1:1 by weight) 
in order to shorten the period needed for 
the clays to dewater sufficiently to sup- 
port the sand layer. This resulted in a 
clay-sand mixture in which the clay frac- 
tions changed from 3.5% to 31.5% solids 
in 133 days. 

The results of the tests were favor- 
able, but there were problems. The turn- 
around time for placing each layer of 
clay is long, about 1 month. As each 
layer is relatively thin, this means that 
large areas must be available to store 
the phosphatic clays. The mechanics of 
the operation present problems. The sand 
must be placed in thin (6— in) layers uni- 
formly over a wide area. Some sort of 
special system or machinery would have to 
be devised to cover more than a small 
area. The test pit was small enough that 
this was not a major factor. Slow drain- 
age through the sand layers was also a 
problem, particularly because of the 
large distances (300 to 500 ft) that 
would be involved in an actual operation 
( 46-47 ) . 

ANDCO PROCESS 

ANDCO, Inc. , developed a process for 
dewatering clay tailings , which was 
tested in cooperation with USS Agri- 
Chemicals and IMC. The ANDCO process was 
based upon the use of a proprietary poly- 
electrolyte flocculant identified only as 
A-802, which had been developed after 
lengthy testing. A 2-week test of the 



12 



Phosphatic clays 



Alternate 



(3% to 5% 
solids) 



Sand tailings 




Initial settling pond 



nitial clay settling 





Sand application to cap 
settled clay at 12% to 
15 7o solids 



Second application of 
clay(3 7o to 5% solids) 



Recycle 
water 

FIGURE 5. - Sandwich construction for sand-clay disposal process. 



ANDCO process was carried out at the USS 
Agri-Chemicals Rockland Plant site at Ft. 
Meade, FL, in September 1973. Besides 
supplying the site, phosphatic clay, and 
sand tailings, USS Agri-Chemicals pro- , 
vided some equipment and labor. The 
clays were pumped into a 30,000-gal 
thickener, the flocculant A-802 was 
added, and the floes were allowed to set- 
tle. At this point, the phosphatic clay 
underflow ranged from 5.5% to 13.1% sol- 
ids. The underflow was pumped to a screw 
conveyor. Tailings sands and additional 
flocculant, A-802 also, were added at the 
input hopper of the screw conveyor. The 
sand-clay mixture was fed into a modified 
drum conditioner. More flocculant was 
added to the drum conditioner as needed 
to "toughen" the agglomerated mixture as 
it passed through. The sand-clay mixture 



was then pumped to a commercial belt 
filter unit. The filter unit used a com- 
bination of pressure and filtration to 
produce a resulting solids content of 55% 
to 70%. Flocculant-laden underflow water 
from the filter unit was recirculated to 
the primary thickener tank. The ANDCO 
process aroused little interest in the 
phosphate industry at the time because of 
what were then believed to be high costs. 
The system is also highly dependent upon 
maintaining a proper ratio of sand to 
clay wastes, which varies, however, with 
the nature of the ore being mined. This 
requires close control and above-average 
operator skills. The process did provide 
rapid consolidation of the clay solids 
and rapid recycling of the process water 
(48-49). 



13 



SAND WICK 

In an effort to expedite the dewatering 
of large masses of clay wastes, several 
types of drainage systems were investi- 
gated. One of the most promising in- 
volved the use of sand wicks or sand 
column drains. Much of the initial theo- 
retical work was done by researchers at 
the University of Florida, supported by 
the National Science Foundation and work- 
ing with the Florida Phosphate Council 
and various phosphate companies. 

In the early efforts, phosphatic clay 
samples were subjected to particle size 
analysis and the permeability character- 
istics were studied. A permeability cell 
was built, and attempts were made to de- 
velop theoretical equations that could 
describe seepage through a sand column. 
Small test cells using sand columns or 
wicks were built and then larger units: 
a 55-gal test cell and a test cell 4 by 4 
by 8 ft high. Field tests, when run on a 
small scale, quickly revealed problems. 
The main one was that the sand columns 
did not retain their shape and cohesion. 
They tended to collapse or to mix with 
the phosphatic clays as the very fluid 
clay migrated. To retain the sand in a 
column, it proved necessary to confine 
it within a rigid but porous structure. 
Perforated PVC pipe filled with sand was 
tested, but burlap sacking within a wire 
screen framework, supported by a small 
pipe and filled with sand, gave the best 
results. This method provided enough 
support so that the sand columns retained 
their integrity, while it also provided 
the most intimate liquid contact possible 
with the clay wastes. 

Large-scale testing was undertaken at a 
site near the IMC Noralyn washer near 
Bartow, FL. Two pits, roughly equal in 
size, were dug. One was the test pit; 
the other was used as a control. The 
test pit was 50 by 50 ft at the base, and 
95 by 95 ft at the top, and 12 ft deep. 
The pit actually extended only about 9 ft 
into the ground because of the high wa- 
ter table; sand tailings were used to 
create a small wall to achieve the de- 
sired total height. On the bottom, a 
1-ft layer of coarse material (3/4-in 



gravel) was spread to provide a drainage 
path. In both pits a sump was fashioned 
from a 3-ft section of steel pipe, 20 in. 
in diam with a 16- by 16-in screened 
opening near the lower end to serve as an 
inlet port. Gravel was piled around the 
sump and then covered with sand to pre- 
vent the clays from entering the sump. A 
plastic pipe was used as a conduit for 
the suction hose connecting the sump with 
a centrifugal pump located on the test 
pit retaining wall. 

In the test pit, 16 sand column "seep- 
age aids" were placed 11 ft apart in a 
square array. These were made of burlap, 
supported by a 1- by 1-in wire mesh cy- 
lindrical casing and were 8 in. in diam 
and 12 ft high. The columns were filled 
with sand tailings , and each had an add- 
tional support, a 1-in aluminum pipe. 
Each was also secured by three guy lines. 

When the sand column seepage aids were 
completed, phosphatic clays were pumped 
into the test pit and into the control 
pit. Some 476,000 gal of 3% solids clays 
were pumped into the test pit and 414,000 
gal of 4% solids clays into the control 
pit. 

After 190 days, the volume remaining in 
the test pit was 67,300 gal of 24% aver- 
age solids (1 ft above the base). There 
were 115,300 gal of 20% average solids in 
the control pit , and there was a foot of 
free water on the surface of the control 
pit, but none on the test pit surface. 
After 280 days, the material in the test 
pit, 1 ft above the base, had reached an 
average of 49% solids compared with only 
28% in the control pit. 

Using reinforced sand columns as seep- 
age aids in dewatering clay wastes ap- 
peared to speed up the dewatering consid- 
erably. There remained some unanswered 
questions. The inability of the sand 
column to support itself meant that rel- 
atively expensive reinforcing systems 
would have to be used to stabilize the 
sand columns. The method was very diffi- 
cult to apply to existing clay settling 
ponds. Attempts were made to establish 
sand column drains by sinking perforated 
PVC pipes into the partially solidified 



14 



clays and then filling them with sand. 
When the pipes were removed, however, the 
sand collapsed or mingled with the shift- 
ing and migrating clay. It also proved 
difficult to slurry out and remove the 
clay material within the pipe so sand 
could be placed there. It proved impos- 
sible to remove sand tailings mixed with 
clay when they were encountered. The 
sand column or sand wick technique also 
suffered from the low permeability of the 
clay, a problem common to all water re- 
moval techniques. The phosphatic clays 
nearest the seepage aids tended to dry 
out, cake, and inhibit the flow of super- 
natant water from farther away. 

On the positive side, the sand column 
or sand wick method did produce more rap- 
id dewatering and resulted in a higher 
percentage solids content for the final 
product. It allowed surface water that 
stood on the pit surface to be drained, 
and it enhanced drying and cracking of 
the clay waste surface, which in turn led 
to further dewatering (50-53) . 

SWIFT, INC., IN-LINE SAND-CLAY MIXING 

Swift, Inc., working with the SCF, un- 
dertook the development of in-line mixing 



of flocculated phosphatic clays and tail- 
ings sands beginning in September 1975, 
as part of the FPCRP. Work was done to 
determine the proper flocculant and the 
optimum sand-clay mixture. It was soon 
found that static, in-line mixers did not 
give the desired results. The sand and 
clay fractions tended to separate, and 
there were problems with inadequate floc- 
culation of the clay fractions. The ef- 
fects of shear during the mixing also 
promoted segregation. These tests led 
Swift (later Estech General Chemicals 
Corp.) to adopt the use of thickeners to 
prethicken the phosphatic clays , a pro- 
cess described below (54) . 

There are numerous variations of the 
general method of using sand tailings to 
mix with dewatered phosphatic clay tail- 
ings in order to both dispose of mine 
wastes and reclaim mined-out areas. 
These different systems are presently 
being evaluated by several phosphate com- 
panies. They differ in some or many par- 
ticulars: for example, the use of spe- 
cial f locculants , the use of special 
thickeners or more than one thickener, 
and the use of special techniques as part 
of the process , such as vibratory or ro- 
tating screens (42, 44). 



TECHNIQUES UNDER CURRENT EVALUATION 



ESTECH SAND- CLAY MIX USING THE 
ENVIRO-CLEAR THICKENER 

Several companies have been and some 
still are experimenting with the use of 
the Enviro-Clear^ clarif ier-thickener to 
dewater clay wastes. The capacity of the 
Enviro-Clear system is greater than that 
of conventional thickeners and is more 
likely to be appropriate for the scale of 
phosphate operations. 

The Enviro-Clear system is a continuous 
solid-liquid separation system. Chemi- 
cally pretreated feed (phosphate clays 
treated with flocculants) is introduced 
into the Enviro-Clear thickener at a 

^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



controlled velocity at the center of the 
unit, as shown in figure 6. The feed is 
introduced horizontally into an "active" 
sludge bed near the bottom of the thick- 
ener. This eliminates the free-settling 
zone, and the flocculated particles ag- 
glomerate and settle to the sludge zone. 
The process is aided by the compres- 
sive force of the thickened sludge bed. 
Continuous rakes aid in compacting the 
solids and moving them to the discharge 
boot. Clear water overflows the top of 
the unit. There is a sharp interface 
between the sludge bed and the clarified 
effluent zone, which is considered char- 
acteristic of the Enviro-Clear device. 
This attribute is used to control the 
system through using either a vertical 
sight glass or an ultrasonic detector 
(55). 



15 



Sand tailings 
from float plont 




FIGURE 6. - Enviro-CIear clarifier-thickener. 



Gardinier, Inc. , rented a 3-ft-diain 
Enviro-Clear unit for pilot plant studies 
in 1977. Dilute phosphate clays were fed 
from a surge tank by a constant head feed 
so that the flow rate was constant. 
Flocculant was introduced into the feed 
stream between the feed tank and the 
Enviro-Clear thickener. Gardinier engi- 
neers were able to achieve a phosphatic 
clay output of 15.8% solids for a 0.8-lb 
consumption of flocculant per ton of 
(dry) phosphatic clays. When combined 
with tailings at a 1:1 ratio, this pro- 
duced a mixture with 27.7% total solids. 
The water recovered, in-plant, from the 
feed amounted to 87.1% (56) . These ini- 
tial results were encouraging, and sev- 
eral other companies undertook parallel 
investigations using larger thickeners 



(57). An average-size phosphate plant 
processing 60,000 gal/min of clay wastes 
at 3% clay solids would require three 80- 
ft-diam Enviro-Clear thickeners. These 
would produce clay slurries thickened to 
12% to 19% solids depending upon the com- 
position and amount of the clay and 
the amount of flocculant used. Problems 
could be caused by the residual floccu- 
lant in the water removed from the clay 
slurries and recycled for phosphate ore 
processing. The remaining flocculant, 
even in tiny quantities, sometimes tended 
to suppress phosphate flotation. All of 
these expensive investments failed to in- 
crease dewatering beyond what could be 
ultimately achieved in conventional set- 
tling ponds over a longer period of time 
(15 to 30 months) (43). There have also 



16 



been difficulties reported in pumping the 
flocculated mixture. 

The Enviro-Clear process has the advan- 
tage of rapidly dewatering dilute phos- 
phatic clays, and it allows clay and sand 
wastes to be stored together in the same 
area. It produces a relatively stable 
and fertile landfill for revegetation. 
It has the disadvantage of being strongly 
dependent upon a certain range of sand- 
to-clay waste mixture. This is an uncon- 
trollable factor, which depends upon the 
nature of the matrix being mined at a 
particular time. There is also the prob- 
lem of residual flocculants in the efflu- 
ent water, which is recirculated for mine 
and plant processing. Remnant flocculat- 
ing agents may render the water unusable. 

Recently, Estech built a plant based on 
the Enviro-Clear thickener, which is cap- 
able of handling the entire waste clay 
output of their Watson Mine. Estech 
chose the Enviro-Clear system because it 
fit well with the Estech mining system, 
used equipment familiar to the operators, 
gave a rapid return to operations of pro- 
cess water, and was cost competitive with 
conventional disposal methods. The Es- 
tech plant went into operation in October 
1981. It is based upon an 85-ft-diam 
Enviro-Clear rapid clay thickener capable 
of handling 348 ton/h of solids (clay 
plus sand tailings) with a feed flow of 
12,000 to 15,000 gal/h. The clay slur- 
ries average 3% to 8% solids. Dewatered 
sand tailings and the clay are mixed in 
approximately equal quantities with an 
anionic polymer in a large mixing tank. 
The mixture is then fed into the Enviro- 
Clear rapid clay thickener. Another mix- 
ing tank is provided in the process 
stream so that more dewatered sand tail- 
ings can be added to the thickened clay 
underflow as an alternative. Pumps move 
the thickened sand-clay underflow out to 
final disposal areas. Clear water over- 
flows the thickener and is returned to 
plant operations. 

To date, Estech is pleased with the 
operation of its Enviro-Clear thicken- 
er and plant. It has been able to get 



sufficient flocculation of the suspended 
clay particles to achieve a solids con- 
tent of over 12% for a flocculant usage 
of less than 1 lb/ton (clay solids ba- 
sis). Mixed with the sand tailings, this 
creates a homogeneous mixture that re- 
mains solid and does not allow segrega- 
tion of the sand and clay fractions. 
Additional consolidation and dewatering 
in the final storage areas has exceeded 
initial expectations. The rapid return 
of process water to plant operations has 
resulted in "significant" savings in 
power costs for pumping. The resulting 
sand-clay mixture is also closer to nor- 
mal surface soil than that produced by 
any other waste clay settling system and 
will result in a more rapid return of 
disturbed land to premining usage, ac- 
cording to Estech (57) . 

There are problems, however. The con- 
stitution of the phosphate matrix varies 
widely from mine to mine and even within 
mines. In some cases there may be insuf- 
ficient sand tailings available to mix 
with the prethickened phosphatic clay 
tailings. In other cases there are large 
quantities of clay fractions present of 
the type that cannot be quickly or eco- 
nomically dewatered. In any case, the 
resulting landfill has low shear strength 
and is only marginally useful, after rec- 
lamation, for any purpose other than 
agricultural. The presence of residual 
flocculants in the reclaimed water may 
inhibit its use when recycled for plant 
operations. 

DREDGE-MIX PROCESS 

IMC undertook the original testing of 
the dredge-mix method around 1975. In 
1978, IMC and Agrico Mining Co., later 
joined by Mobil Chemical Co., undertook 
jointly sponsored research on waste clay 
consolidation methods and land reclama- 
tion. The dredge-mix process was de- 
signed to mix tailings sand with already 
naturally thickened clay wastes, which 
would be constantly pumped from the bot- 
tom of the initial settling pits (fig. 
7) . The testing was to determine if this 



17 



Holding pond, 
dredging 




Sand tailings (30 % solids) 

Q— "^ 



Cyclone 
-> — ' 



Dewatered 
sands 




Prethickened 
clays 



Recycle 



water 



Thickened clays 




Disposal area 

FIGURE 7. - International Minerals and Chemical Corp. process for clay disposal. 



sand-clay mixture would produce land suf- 
ficiently consolidated for multipurpose 
uses. 

Based upon research carried out at the 
Colorado School of Mines , a Marcona-type 
pump (Marconaflo) was used, which was 
equipped with a high-pressure rotating 
water jet near the intake of the sub- 
merged pump. The water jet proved super- 
fluous, having little effect on the 
pumping of the clay wastes. Also, the 
Marconaflo pumping system was costly, and 
it was soon replaced by conventional sub- 
merged pumps. These proved capable of 
pumping the partially dewatered clay 
wastes as long as the solids content was 
under 21%. It also proved necessary to 
cover the intake of the submerged pump 
with about 8 ft of clay wastes exceeding 
15% solids to prevent the preferential 
development of channels through lower 
density material. 

Mixing sand tailings with the thickened 
clay wastes was soon abandoned by the 
IMC-Agri co-Mobil group when it was dis- 
covered that the addition of the sand did 



little to speed up the process of dewa- 
tering. Laboratory tests carried out by 
Mobil confirmed this. 

At a later time, interest in the 
dredge-mix process was renewed, and CF 
Mining Corp. began a full-scale sand-clay 
mix reclamation project at its mine. The 
project was overseen, initially, by Arda- 
man and Associates , under a research 
grant from FIPR. The main objectives of 
the tests were to obtain data on the set- 
tling and consolidation of the sand-clay 
mix and to evaluate the nature and qual- 
ity of the reclaimed land ( 58 ) . 

W. R. Grace and Co., at its Four Cor- 
ners Mine, has committed itself to build- 
ing a pilot dredge-mix settling area. It 
is planned to dredge prethickened clay 
from the initial settling area, mix it 
with sand tailings , and place it in 
mined-out cuts. This is approximately 
the same process originally attempted by 
the IMC-Agri co-Mobil research group (43) . 

The greatest advantage of the dredge- 
mix system is the ability to use it with 



18 



a thickener to produce clay wastes of 
sufficient density to allow their rapid 
settling when mixed with sand tailings. 
The greatest difficulty is dependence of 
the process upon certain specific sand- 
to-clay ratios, when the mined matrix is 
actually highly variable in content (43) . 

A comprehensive comparative evaluation 
of sand-clay mix phosphatic clay disposal 
compared with disposal in conventional 
settling ponds has been undertaken by 
Ardaman and Associates , under a research 
grant from FIPR. The aim was to deter- 
mine quantitatively and qualitatively the 
advantages and disadvantages of each dis- 
posal method and to determine the engi- 
neering properties of a range of sand- 
clay mixes and of various clays. From 
this , it is hoped a mechanism can be de- 
veloped for prediction of sand-clay mix 
properties based upon the mineralogical 
and settling characteristics of various 
clays. The study was divided into three 
parts. In the first, the index proper- 
ties of the phosphatic clays were deter- 
mined. On 12 phosphatic clay and 3 sand 
tailings samples, selected to be repre- 
sentative of the whole phosphate area, 
soil mechanics index tests were run. 
These included plasticity characteristics 
(Atterberg limits) , particle size distri- 
butions, specific gravity, activity, and 
viscous properties. The pH and specific 
conductance of the pore fluid were also 
measured. The samples were found to be 
within the range and distribution of pre- 
viously published values. Mineralogical 
tests were also run, which included X-ray 
diffraction, scanning electron micro- 
scopy, and chemical analysis. From these 
data, quantitative estimates were made of 
the mineral species. It was found that 
clays such as kaolinite, montmorillonite, 
and attapulgite comprised from 40% to 65% 
of the phosphatic clays. Sedimentation 
tests were done on the 12 clay and 3 sand 
tailings samples. Some 22 laboratory 
settling tests were run. From these, the 
final settled solids content, settling 
rate, and void ratios versus effective 
stress relations at low stress were de- 
termined. The data were found to be con- 
sistent with the Michaels and Bolgar 



theory of settling behavior of clays. 
Few correlations were found, however, be- 
tween index properties and sedimentation 
behavior (59-60). 

DREDGE PROCESS 

The IMC-Agrico-Mobil research effort 
redirected its attention from the dredge- 
mix process to the placement, by pumping, 
of the thickened clay wastes (15% to 18% 
solids content) in mined-out pits, and 
covering them with an evenly distributed 
sand cap. This has been labelled the 
dredge process by IMC. Lawver of IMC had 
calculated that it would generally re- 
quire an effective stress of 600 to 800 
Ib/ft^ on the clay wastes to dewater them 
sufficiently, over time, to return them 
to near original volume. This would al- 
low the clay to be placed below ground 
level but would still require an overbur- 
den cap above the original surface con- 
tour. Placing a uniform sand cap on the 
clay wastes proved difficult. There were 
numerous failures before what appears to 
be a workable system was developed. Be- 
tween 1978 and 1982, IMC, Agrico, and 
Mobil undertook sand spray tests to place 
a cap, under the direction of Lawver. 
The first attempt to use a sand spray 
nozzle developed by Brewster Phosphates 
failed. The nozzle was buoyed with 
floats to maintain its position near the 
surface. Sand tailings at approximately 
30% solids were sprayed on the test pond 
surface at a rate of 1,000 ton/h. The 
spray nozzle was unable to deposit the 
sand uniformly, to form a cap; the sand 
layer, ranging from to 2 ft in thick- 
ness, was heaviest near the spray. This 
unevenness caused the heavier sections to 
sink and created heaving and mud waves. 
Mud waving occurs when the clay surface, 
unable to bear the weight of the load im- 
pressed on it, either flows plastically 
from under the load or collapses and al- 
lows the sand load to sink, usually by 
sections, where the impressed load is 
heaviest. The term is used to describe 
movement of the impounded clay-sand mix- 
ture as induced shear stresses set up a 
heaving wave front during redistribution 
of the clay-sand solids. 



19 



Another attempt to deliver sand from 
the sides of the pond also failed. The 
sand was deposited only near the outlets, 
with consequent unevenness. A central 
mud wave was also created (43) . In an 
attempt to solve the problem of the even 
distribution of a sand cap, research at- 
tention turned toward the development of 
a surface material capable of bearing the 
weight of a sand cap. 

Systematic tests were undertaken to 
develop a surface covering, natural or 
artificial, with a greater bearing 
strength. This would allow a greater 
depth and weight of sand to be placed on 
the clay wastes. Japanese millet was 
planted in one test. It thrived and 
within 10 weeks had produced a thick 
mass, 2 ft high with root systems extend- 
ing 1-1/2 ft down, which took over the 
entire pit and naturally reseeded itself. 

The use of a flexible fiber covering 
(Du Pont Typar spun woven fabric) as a 
base for the sand cap was also tested. 
Problems developed when the flexible 
hoses pumping the sand wore through the 
Typar fabric and the sand was deposited 
below the surface. In another area where 
the concentration of the sand tailings 
was too high, the Typar fabric failed and 
the sand tailings passed into the clay. 
An attempt was made to place a Typar mat 
on the surface of the clay wastes and 
then to cover it with sand tailings from 
one side, progressing across the pit by 
the addition of new strips of Typar. 
This failed also because the sand tail- 
ings remained concentrated instead of 
flowing across the Typar surface. As 
they sank, the concentrated sand tailings 
pushed the fluid clay wastes out as a mud 
wave . 

Another test pit was filled with a sand 
and clay slurry with a 3:2 sand-to- 
clay ratio and with an average solids 
content of 26.9%. The pit was covered 
with a growth of cattails, except for one 
small portion where Japanese millet was 
planted, which soon covered that area. A 
Typar strip was laid down near the edge 
and covered with sand tailings. A small 
bulldozer worked the tailings into a thin 



(2 ft) layer covering a wider area. Ty- 
par strips were then added, and more sand 
tailings deposited, to be worked into a 
thin layer by the bulldozer, working from 
the sides of the pit. This finally pro- 
duced a 6-ft high mud wave of thickened 
clay in the pit center. But this too was 
successfully covered by the Typar and 
sand tailings. After the 2-ft sand tail- 
ings cap was completed, additional sand 
tailings were added to produce a sand cap 
averaging 6 to 8 ft in thickness. Once 
the sand cap was in place, additional de- 
watering and consolidation of the clay 
wastes continued. The tests achieved the 
expected results but seem unlikely to re- 
sult in a workable, industrial-scale pro- 
cess because of the high equipment and 
labor costs. 

Typar fabric is also being used in 
another test. High-ratio (4.9 parts sand 
to 1 part clay) sand-clay mix was pumped 
as a slurry onto the Typar mat. The mix- 
ture was easily and accurately delivered 
in a uniform sand cap without any failure 
of the Typar fabric. Corrugated metal 
sheets were placed below each outlet to 
minimize the impact of the incoming slur- 
ry upon the slurry that was already set- 
tling. Underneath the cap, the desired 
consolidation is progressing. 

The research is continuing. There are 
questions of to what extent laboratory 
results can be duplicated in the field. 
In particular, there are questions of 
whether the effective stress of the sand 
cap will be dissipated through the ex- 
pected 20- to 40-ft (or more) depth of 
the clay wastes and how far water can 
move through the clay. The IMC-Agrico- 
Mobil research project is still in prog- 
ress. Tests are ongoing on a pit con- 
taining 27,000 tons of clay and 40,500 
tons of sand that has recently been 
capped with 7 ft of tailings ; a second 
pit has only clay wastes, 47,250 tons, at 
21% solids content, 40 ft deep, but it 
has not yet been capped; a third shallow 
pit contains clay wastes at 18% solids 
content, covered with a Typar mat and 
capped with 1-1/2 ft of sand and clay in 
the ratio of 4.9:1 (45). 



20 



The greatest advantage of the dredge 
process is that It has the ability to 
dispose of both the sand and clay tail- 
ings in the same disposal site and to 
create a relatively stable landfill at 
the same time. The resulting surface is 
fertile for revegetation and agricultural 
purposes. The greatest disadvantage lies 
in the logistics difficulty of providing 
the needed ratio of sand and clay at the 
time they are required. This ratio nor- 
mally depends upon the material in the 
matrix being mined and is not subject to 
easy adjustment by mine operators. 

SAND SPRAY PROCESS 



Standby 



Phosphatic clays 



(3% to 5% solids) 



^--S 




Initial clay settling 



M^; i I 




Sand tailings (30% solids) 
Disposal area ^ 



Sand capping I 



Recycle 



water 



FIGURE 8, - Sand spraying process for clay di sposalt 



Sand spray reclamation techniques grew 
out of early (1969 and before) tests of 
sand-clay mix disposal experiments. It 
was found, at the Brewster plant of Amer- 
ican Cyanamid Co. (now Brewster Phos- 
phates) , that sand-clay mixes showed sep- 
aration of the sand and clay when the 
mixtures were pumped into a settling 
pond. The sand fraction settled quickly 
while the clay fraction migrated and only 
settled slowly. It was found, however, 
that the sinking sand fell "gently" on 
thickened clay already in the pit. The 
sand migrating through the clay liberated 
large quantities of water and left behind 
clay solids that, over a period of time, 
approached 35% solids. This led to the 
concept of sand capping ( 62 ) . 

After small-scale experiments proved 
successful, large-scale tests of sand 
spray techniques were tested in 1974 and 
1975. Figure 8 shows a flowsheet of the 
process. At Brewster's Haynsworth Mine, 
five large cuts, typically 350 ft wide, 
30 to 40 ft deep, and 2,000 ft long, were 
used for tests. Phosphatic clay slurries 
were pumped into the cuts and allowed 
to settle until they had reached ap- 
proximately 13% solids. Then they were 
sprayed with sand from plant tailings. 
The sand was applied through a floating 
pipeline fitted with spray nozzles. The 
entire apparatus was movable to allow 
accurate and even placement of the sand 
over the clay wastes. The sand was 
sprayed on the clay at a rate of 825 



ton/h. Two separate passes of the sand 
spray were made, and each resulted in 
"significant" dewatering. The total sol- 
ids content (for two passes) rose from 
13% to 60%, and for the fine clay frac- 
tion (minus 150 mesh) , it rose from 13% 
to 25% (23). 

Occidental Chemical Co., under an FPCRP 
program, tested sand spray techniques at 
a 5-acre test site located at the White 
Springs Mine in 1974 and 1975. Approxi- 
mately 20,500 tons of waste clay solids 
in the test pit had been sand-capped in 
August and September 1974. An additional 
4,500 tons of clay waste averaging 12.2% 
solids were added. The overall solids 
content was approximately 21%. In a pe- 
riod of 4.5 h, some 4,300 tons of sand 
were sprayed over the settled clay-sand 
already in the pit. It was found impos- 
sible to add sand beyond a ratio of 1:5, 
sand to phosphatic clay. When this was 
exceeded, large-scale mud waving oc- 
curred. It proved difficult to spray the 
sand uniformly on the clay surface. The 
uneven weight caused the surface to 
heave. The sand spray test results were 
disappointing; the clay wastes in the 
top, recently added layer, increased from 
12.2% to 15.8%, but the overall value 
for the fine clay material (minus 150 
mesh) for the entire test pit remained 
unchanged. It was concluded that the 
"technique was not feasible under the 
conditions tested" (44). On the other 
hand, boreholes made 2 yr later revealed 



21 



that the clays had continued to lose wa- 
ter and had reached 30% to 40% solids. 
The total solids content (sand and clay) 
ranged from 50% to 70%. This left a soil 
that provided sufficient strength for 



agricultural purposes (animals and vehi- 
cles). High pore pressures were also en- 
countered, indicating that further dewa- 
tering could be expected through time 
(39). 



DEVELOPING TECHNOLOGY 



GARDINIER, INC., PROCESS 

Gardinier, Inc. , undertook tests from 
1976 to 1979 of small pilot units of clay 
thickeners manufactured by six different 
companies. With equipment produced by 
the French company, Alsthom Atlantic, a 
sludge-bed- type, clay waste thickener pi- 
lot plant was designed and built. It was 
operated in 1980 and 1981. Gardinier and 
Alsthom Atlantic found that the (then) 
state-of-the-art flocculating and thick- 
ening pilot plant "did not increase dewa- 
tering of (clay wastes) beyond that which 
could be obtained by settling in... ponds" 
(44). 



clarified water moves upward where it is 
removed. The thickener is funnel shaped; 
the concentrated sludge is removed at a 
bottom outlet at a controlled rate to 
maintain the desired density. The floc- 
culant used is a common one (unnamed) , 
which Gardinier intends to manufacture 
itself (63-64). 

Gardinier is building a large-scale 
test unit utilizing its process. Costing 
over $20 million, the system reportedly 
will handle all of the phosphatic clays 
generated by Gardinier 's Ft. Meade, FL, 
mine. It was scheduled to begin service 
in June 1983 (44). 



I 



Further research by Gardinier and Als- 
thom Atlantic resulted in the develop- 
ment of a dewatering process for phos- 
phatic clays called "superflocculation" 
or "aqueous agglomeration" or "pellet 
f locculation. " While the details have 
not been publicly released, it appears 
that the Gardinier process is essentially 
a two-stage flocculation and thickening 
process centered on Alsthom Atlantic's 
Clariflux thickener. 

The Clariflux unit acts as a floccula- 
tor, a clarifier, and a thickener. It is 
an entirely static unit utilizing an 
active sludge bed to concentrate floes. 
Untreated feed passes into the center of 
the unit, as shown in figure 9, where it 
is flocculated. Large floes pass down- 
ward through an orifice into the thicken- 
er. The remainder moves upward into the 
clarifier unit with decreasing velocity 
into an active sludge bed. The slowing, 
small floes agglutinate with the larger 
floes already present. Size and density 
of the floes increase, the enlarged floes 
pass downward to the thickener, and 



Untreated 
clay slurry 




3. Clarified 
S' water 



Concentrate 
clay solids 



FIGURE 9. - Clariflux thickener. 




22 



The test unit will take the phosphatic 
clay slurries from the phosphate plant 
averaging 1% to 6% clay solids con- 
tent and pump them to five huge Clari- 
flux thickeners (fig. 10). There the 
clays will be flocculated and thickened. 
The thickened clay underflow comprising 
12% to 15% solids will be mixed with 
sand tailings and pumped to the previ- 
ously mined-out cuts. Here will be lo- 
cated a "superf locculation station" where 
additional flocculant will be added. 
Large floes form at this stage and when 
emplaced in the mining cuts reportedly 
dewater rapidly to near 25% solids in 
24 h. The dewatering continues in the 
pits, and a solids content of 30% to 40% 
is reached in a few weeks, as reported 
by Gardinier. After this dewatering, 
more thickened clay-sand tailings will 
be placed in the pit in stages , to al- 
low each addition to dewater before 
more is added. After approximately 1 yr, 
the sand-clay mixture in the pits will 
be capped with additional sand tailings 
and overburden. Gardinier anticipates 
that this will restore the mined-out 
areas to approximately the original con- 
tour before mining began. The entire 
process reportedly will require only 3 
to 5 yr. 



Gardinier believes that its process 
represents the "best available technol- 
ogy." It will restore the land to pre- 
mining agricultural uses as soon as pos- 
sible and presents a method of disposing 
of clay wastes below grade. This elimi- 
nates the danger of flooding and pollu- 
tion from dam breaks. It also quickly 
frees large quantities of water, which 
will result , Gardinier believes , through 
better water management, in lower usage 
of water from the ground, thereby leaving 
more for other competitive uses. 

Gardinier does not claim to have found 
the answer to the clay disposal problem; 
it claims only to have found the answer 
to its clay disposal problem. Gardinier 
points out that the process may not work 
for different ores from different mines 
and recommends that "extensive" investi- 
gations be undertaken before attempts are 
made to apply the Gardinier process to 
other ores and other conditions. Each 
situation may prove to be unique. Never- 
theless, Gardinier is so confident of the 
success of its process that it is risk- 
ing an investment larger than the total 
original 1967 cost of the entire mine, 
equipment, and processing plant on its 
outcome. 



Recycled water 



Phosphatic ■ ^, ... . i 

rinuc; ' Clarified I 

(3% to 5% Flocculant water Flocculant 

solids) 




Thickened 



Pump 



Pump solids 



i Sand tailings I 




r- Disposal areas' 




FIGURE 10. - Gardinier, Inc., process for clay disposal. 



23 



The greatest advantage of the Gardinier 
process is the rapidity with which the 
clay wastes can be dewatered and the con- 
sequent result that the total volume can 
be stored at or near ground level, elim- 
inating storage ponds. It has, however, 
not been tested on a full industrial 
scale (44, 63-64). 

BUREAU OF MINES ROTARY TROMMEL 

The Bureau, as part of the FPCRP, un- 
dertook after 1972 the systematic test- 
ing of several types of reagents for 
possible use as clay settling agents. 
Nitrogen-bearing, sulfur-bearing, inor- 
ganic, and many commercially available 
reagents were tested for their ability 
to precipitate suspended clay particles. 
The reagents were tested in the labora- 
tory against a representative cross sec- 
tion of phosphatic clay slurries from 
several different mines in central and 
northern Florida. The reagents were also 
tested at varying concentrations (0.1, 
0.01, and 0.001 M) and at varying pH lev- 
els. Neither the reagents, their concen- 
tration, nor the pH of the slurries had 
any significant effect upon the clay sus- 
pensions, with two exceptions. 

Hydrofluoric acid (HF) caused the sus- 
pended clay particles to settle out vig- 
orously, particularly the attapulgite 
clay fraction. Other fluorine compounds, 
such as hydrofluosilicic acid and ammo- 
nivim fluoride, also caused the suspended 
clay to precipitate. However, because 
of expense and environmental considera- 
tions, the use of fluorine compounds was 
not considered feasible for treating clay 
discharge. 

One commercially available reagent 
caused an unusually strong flocculation 
and dewatering effect in attapulgite sam- 
ples and even a stronger reaction with 
montmorillonite and other plant phos- 
phatic clays. The reagent was polyethy- 
lene oxide (PEO), a water-soluble, non- 
ionic polymer with a molecular weight of 
5 to 8 million. PEO has the approximate 
formula (CH2-CH2-0)x. 



Because of its promise, further tests 
were run on the PEO-clay interaction. It 
was found that there was no correlation 
between the composition of the clay slur- 
ry and the PEO needed to dewater it. 
Each clay sample had to be evaluated in- 
dividually, and the samples exhibited a 
wide range of consumption of PEO when de- 
watered. Laboratory tests were also un- 
dertaken to determine the effects of PEO 
concentration on five samples of mine 
clay wastes. The concentrations tested 
were 0.25%, 0.10%, 0.05%, 0.01%, 0.005%, 
0.0025%, and 0.001%. It was found, gen- 
erally, that lower concentrations re- 
quired the use of less reagent, but at 
extremely low concentrations, so much 
more water was being added to the slurry 
that it made the dewatering even more 
difficult. Tests were also carried out 
to determine the effect of varying solids 
content in the phosphatic clay feed. 
Generally, it was seen that an increased 
amount of reagent was required with an 
increased percent solids content of the 
clay slurry feed. 

Experiments were also done to create a 
continuous process using PEO. A horizon- 
tal vibrating screen dressed with 100- 
mesh wire cloth was tested and success- 
fully dewatered the clay waste-PEO slurry 
to 29% solids. A curved, static, wedge- 
wire screen was used to dewater a 3.9% 
solids phosphatic clay feed to 17% sol- 
ids. This in turn was dewatered further 
in tests, to 31% using a horizontal vi- 
brating screen and to 27% using a rotat- 
ing trommel. A screw classifier success- 
fully dewatered 3.9% solids clay slurry 
to 30% solids. A rotating trommel (0.38 
m in diam, 0.9 m in length) fitted with 
10-mesh wire screen was also used in 
tests. In the trommel, the clay-PEO 
slurry, previously mixed, dewatered very 
rapidly and formed consolidated rolls 
or masses , as it moved through the 
trommel, of 25% to 30% solids phosphatic 
clay. The discharge water, however, con- 
tained some flocculated solids that 
leaked through the first 25 to 30 cm of 
the trommel screen. 



24 



Other small-scale tests investigated 
the effects of such variables as trommel 
speed, trommel slope, reagent concentra- 
tion and dosage, retention time, feed 
rate, and variations in clay slurry feed 
and reagent mixing (65) . 

The success of small-scale dewatering 
of phosphatic clays with PEO and a rotary 
trommel led to tests on a larger scale. 
A field test unit, designed to treat 100 
gal/min of dilute phosphatic clays, was 
fabricated and operated at Estech General 
Chemicals Corp.'s Silver City Mine (fig. 
11). The trommel screen of the field 
test unit was built of 10-mesh stainless 
steel wire screen. It was 24 ft long and 
3 ft in diam. Later the trommel was 
shortened to 12 ft, and a static screen 
or hydrosieve was added. The first 6 ft 
of the feed end of the rotary trommel was 
also lined with 48-mesh stainless steel 
screen to prevent premature loss of fine 



particles of agglomerate. The clay slur- 
ries were pumped into a mixer at rates 
from 60 to 124 gal/min and then into the 
trommel, or into the hydrosieve screen 
and then into the trommel. The dewatered 
clays exited the trommel at 14% to 20% 
solids and were pumped to a pit where de- 
watering continued. 

The large-scale tests were carried on 
intermittently over an 18-month period. 
Wide variability in the nature of the 
waste clay feed was soon observed. Plant 
operators reported that a fringe area was 
being mined and that the mined material 
was not typical of normal phosphate de- 
posits in the area. The variability of 
the clay waste composition caused many 
problems for operations of the field test 
unit. Lime had to be added to sev- 
eral samples to oxidize the noncalcium 
exchanged clay. 



Plant 




Flocculant 
(PEO) 



Hydrosieve 



Mixing tanl( 



Rotary trommel 



Recycle 
water to plant 




Slime solids (247o) 
to mine cuts 



///MWW/VMW 




Disposal area 



FIGURE n. - Bureau of Mines rotary screen dewatering system. 



25 



Results of the large-scale test unit 
were satisfactory. The solids content of 
the dewatered clay slurries ranged from 
14.3% to 16.0% for a reagent usage of 3.0 
lb/ton of PEG at 0.25% concentration to 
1.3 lb/ton PEG at 0.025%. When the PEG 
dosage was increased, it did not produce 
significantly higher solids content in 
the dewatered clay wastes. PEG dosages 
as high as 6 lb/ton improved the solids 
content to only about 16.2%. 

The Silver City Mine clays were effec- 
tively dewatered with lower reagent usage 
by using PEG of about 8 million mol wt. 
This is not necessarily true for any 
other mine phosphatic clays. Using very 
dilute PEG (0.0025% concentration) al- 
lowed the dosage to be reduced to 0.5 lb/ 
ton. But the additional water deleter- 
iously affected the efficiency of 
the trommel. The hydrosieve screen was 
placed between the mixer and the trommel 
to overcome this problem. Clay slurries 
at 3.0% solids were dewatered by the 
hydrosieve screen to about 8.8% solids 
before being introduced into the trommel. 

The field test unit operating at 
100 gal/min, using 0.01% and 0.005% 



concentrations of PEG, dewatered phos- 
phatic clay slurries to 20.6% solids with 
a consumption of 0.69 lb of PEG reagent 
per ton. It was also necessary to use 1 
or 2 lb of lime per ton. 

The dewatered clays were then pumped 
into a pit 15 by 45 by 4 ft deep at 16% 
to 20% solids to test continued dewater- 
ing. Within 30 days, the clays had dewa- 
tered to 30% solids; after 45 days, to 
40% solids. This continued dewatering 
came despite the fact that there was a 
rainfall of 18 in during the first 30 
days the clay samples were in the pit 
(66). 

Further laboratory-scale studies were 
undertaken by the Bureau to determine if 
cheaper substitutes could be found for 
PEG. In particular, several groups of 
reagents were investigated for possible 
synergistic effects when used with PEG. 
Most were found to be of little use; only 
those capable of flocculating the phos- 
phatic clays proved useful. One class of 
reagents, natural guar gums, did produce 
a synergistic effect with PEG, which re- 
sulted in reducing the amount of PEG re- 
quired by 54% (67). 



SUMMARY 



The disposal of phosphatic clays has 
been a problem to the Florida phosphate 
industry that has grown with the industry 
and today represents probably one of the 
mining industry's largest waste-handling 
problems. Although large clay storage 
impoundments have been the most practical 
solution, they are unpleasing aestheti- 
cally, present a possibility of dam fail- 
ure, and require considerable acreage 
that could be used for other purposes. 
Immense amounts of water are tied up in 
the impounded clays that are desired for 
alternative uses. 

Recognizing these problems long before 
the age of environmental awareness, the 
phosphate companies have almost continu- 
ally sought answers to the problem of 
clay dewatering. Millions of dollars and 
uncountable hours have been spent by 
the industry, the Bureau of Mines, and 



private and other governmental research- 
ers on the phosphatic clays problem. The 
companies have vast amounts of money tied 
up in the real estate used for storage 
areas and in the need for replacement wa- 
ter used in mining and benef iciation. 

All of this work has resulted in the 
numerous systems described in this re- 
port. Many methods to dewater the clays 
resulted from these studies. However, 
their technical feasibility has not been 
fully demonstrated; therefore, they have 
not found full-scale adoption in the in- 
dustry. The most common drawbacks have 
been of cost, either prohibitively high 
original investment costs or operating 
costs, particularly energy costs. There 
were also numerous problems of scale. 
Processes that showed promise in the 
laboratory often failed on a large scale. 
New technical problems were created by 



26 



some of the experimental dewatering tech- 
niques, for example, when flocculants 
were added to the clays. But the most 
significant obstacle to the development 
of any universally effective dewatering 
process was the extreme variability of 
the clays themselves. Because of their 
widely variable nature throughout the 
phosphate district and even within one 
mine, successful methods used to dewater 
one suite of clays failed tests on anoth- 
er. Dewatering processes were found to 
be highly site specific. 



While many of the experimental and de- 
veloping dewatering techniques show prom- 
ise of future success and some may even 
be practical now under radically changed 
economic conditions, none can presently 
replace conventional settling in terms of 
cost and efficiency. Several of the de- 
veloping techniques described are already 
being incorporated into conventional pro- 
cesses where they are used along with 
conventional settling processes to speed 
the rate of initial dewatering. 



REFERENCES 



1. Opyrchal, A. M. , and K. L. Wang. 
Economic Significance of the Florida 
Phosphate Industry. BuMines IC 8850, 
1981, 62 pp. 

2. Environmental Science and Technol- 
ogy. Those Nasty Phosphatic Clay Ponds. 
V. 8, Apr. 1974, pp. 312-313. 

3. Terichow, 0., and A. May. Electro- 
phoresis and Coagulation Studies of Some 
Florida Phosphate Slimes. BuMines RI 
7816, 1973, 8 pp. 

4. Moulik, S. P., F. C. Cooper, and 
M. Bier. Forced-Flow Electrophoretic 
Filtration of Clay Suspensions. Filtra- 
tion in an Electric Field. J. Colloid 
and Interface Sci., v. 24, 1967, pp. 427- 
432. 

5. Speil, S., and M. R. Thompson. 
Electrophoretic Dewatering of Clays. 
Trans. Electrochem. Soc. , v. 81, 1942, 
pp. 119-145. 



8. Mitchell, J. K. In Place Treat- 
ment of Foundation Soils. J. Soil Mech. 
and Found. Div. , ASCE, v. 96, No. SM-1, 
1970, pp. 73-110. 

9. Stanczyk, M. H. , and I. L. Feld. 
Electro-Dewatering Tests of Florida Phos- 
phate Rock Slime. BuMines RI 6451, 1964, 
19 pp. 

10. Lawver, J. E. (assigned to Int. 
Miner, and Chem. Corp., Chicago, IL) . 
Electrostatic Benef iciation of Non- 
Metallic Minerals. U.S. Pat. 2,754,965, 
July 17, 1956. 

11. Pohl, H. A., and J. P. Schwar. 
Particle Separations by Nonuniform Elec- 
tric Fields in Liquid Dielectrics. 
Princeton Univ. Plast. Lab., Princeton, 
NJ, Tech. Rep. 53B, Mar. 25, 1959, 12 pp. 

12. U.S. Bureau of Mines. The Florida 
Phosphate Slimes Problem. A Review and a 
Bibliography. IC 8668, 1975, 41 pp. 



6. Sprute, R. H. , and D. J. Kelsh. 
Laboratory Experiments in Electrokinetic 
Densification of Mill Tailings (In Two 
Parts). 1. Development of Equipment and 
Procedures. BuMines RI 7892, 1974, 
77 pp. 

7. . Laboratory Experiments in 

Electrokinetic Densification of Mill 
Tailings (In Two Parts). 2. Application 
to Various Types and Classifications of 
Tailings. BuMines RI 7900, 1974, 43 pp. 



13. Davenport, J. E., G. W. Kieffer, 
and E. H. Brown. Disposal of Phosphate 
Tailing. Div. Chem. Dev. , Res. Branch, 
Tenn. Val. Auth., Wilson Dam, AL, TVA 
Rep. 661, July 1953, 125 pp. 

14. Houston, E. C, V. C. Jones, and 
R. E. Powell. Dewatering of Phosphate 
Tailings. Trans. AIME, v. 184, 1949, 
pp. 365-370. 



27 



15. Kelly, H. J., and H. M. Harris. 
Electrical Dewatering of Dilute Clay 
Slurries. BuMines RI 6479, 1964, 21 pp. 

16. Kelland, D. R. High Gradient Mag- 
netic Separation Applied to Mineral Ben- 
eficiation, Pres. at 1973 Intermagn. 
Conf. (paper 80, cat. 5,901,000), 1973, 
21 pp. MIT Francis Bitter Nat. Magn. 
Lab., Cambridge, MA. 

17. Thompson, D. Ultrasonic Coagula- 
tion of Phosphate Tailings. Bull. Va. 
Polytech. Inst., Blacksburg, VA, Eng. 
Ser. 75, v. 63, No. 5, July 1950, 77 pp. 

18. Murdock, H. R. Industrial 
Wastes, Florida; Cull Citrus Fruit and 
Phosphate Rock Slimes Could Solve Each 
Other's Problems. Ind. and Eng. Chem. , 
V. 44 (suppl.), 1952, pp. 115A-116A, 
118A. 



Slime Dewatering. Paper in Fine Parti- 
cles Processing (Proc. Int. Symp., AIME, 
Las Vegas, NV, Feb. 24-28, 1980). AIME, 
1980, pp. 1000-1011. 

25. Woodward, F. E., and L. Gustafson. 
A Survey of Available Flocculants for 
Use With Phosphatic Clay Slimes. Surf, 
Chem. of FL, Inc., Miami, FL, Oct. 10, 
1974, 53 pp. 

26. Hamer, M. , and 0. F. Batzer (as- 
signed to Int. Miner, and Chem. Corp., 
Chicago, IL) . Use of Chelating Agents 
for Enhancing the Settling of Slimes. 
U.S. Pat. 4,049,547, Sept. 20, 1977. 

27. Tarrer, R. , and F. Kleinschrodt. 
Quarterly Report of Phosphate Slime Re- 
search Team (BuMines Cooperative Fellow- 
ship Program). Auburn, Univ., Auburn, 
AL, Nov. 22, 1974, 3 pp. 



19. Greene, E, W. (assigned to Miner. 
Sep., N. Am. Corp., New York). Concen- 
tration of Land Pebble Phosphate. U.S. 
Pat. 2,571,866, Oct. 16, 1951. 



28. Tarrer, R. , and J. Hardiman. Bu- 
reau of Mines Cooperative Fellowship Pro- 
gram: Progress Report. Auburn Univ., 
Auburn, AL, June 2, 1976, 6 pp. 



20. . (assigned to Miner, and 

Chem. Philipp Corp., Menlo Park, NJ) . 
Phosphate Matrix Benef iciation Process. 
U.S. Pat. 3,302,785, Feb. 7, 1967. 

21. Buie, B. F. , and T. J. Fellers. 
Electron Microscope Investigation of the 
Shape and Texture of Particles and Aggre- 
gates as a Factor in Their Flocculation 
and Settling in Phosphate Slimes (con- 
tract HOI 33076, FL State Univ.). BuMines 
OFR 52-76, 1975, 135 pp. 

22. McConnell, C. L. Mineral Varia- 
tions in Phosphatic Slimes. M.A. Thesis, 
Univ. S. FL, Tampa, FL, 1973, 65 pp. 

23. Bromwell, L. G. Annual Report, 
1975. Florida Phosphatic Clays Research 
Project. FL Inst. Phosphate Res., Bar- 
tow, FL, Jan. 1976, 25 pp. 

24. Onoda, G. Y, , Jr., D. M. Deason, 
and R. M. Chlatre. Flocculation and Dis- 
persion Phenomena Affecting Phosphate 



29. Florida Institute of Phosphate Re- 
search (Bartow, FL). Use of Colloidal 
Gas Aphrons in Treatment of Phosphate 
Slimes. Proj . Sum., Proposal 82-02-027, 
1982, 1 p. 

30. . A Study of the Influence 

of Lime Addition on Waste Phosphatic 
Clay Dewatering and Stabilization. Proj. 
Sum., Proposal 82-02-024, 1982, 1 p. 

31. . Waste Clay Disposal in 

Mine Cuts. Proj. Sum., Proposal 81-02- 
006, 1981, 1 p. 



32. 
ties of 



Physico-chemical 



Proper- 
Florida Phosphatic Clays. Proj. 
Sum., Proposal 80-02-003, 1980, 1 p. 



33. Brierley, C. L., and G. R. Lanza. 
The Microbiological Flocculation of 
Phosphate and Potash Slimes (contract 
J0199150, NM Inst. Min. and Tech.). Bu- 
Mines OFR 195-82, 1982, 131 pp.; NTIS PB 
83-137315. 



28 



34. Stanczyk, M. H. , and I. L. Feld. 
Chemical Processing of Florida Phos- 
phate Rock Slime. BuMines RI 68A4, 1966, 
11 pp. 

35. Vondrasek, A. F. A Summary Eval- 
uation of Slimes Disposal Techniques. 
Int. Miner, and Chem. Corp, Mulberry, FL, 
Internal rep., June 1962, 67 pp. 

36. Korpi, G. K. Electrokinetic and 
Ion Exchange Properties of Aluminum Oxide 
and Hydroxides. Ph. D. Thesis, Stanford 
Univ., Stanford, CA, 1965, 132 pp. 

37. Lewis, D. R. Ion Exchange Reac- 
tions of Clays. Paper in Proc. 1st Natl. 
Conf. on Clays and Clay Technology. 
Bull.-Calif . , Div. Mines and Geol. , 169, 
July 1955, pp. 54-69. 

38. Stanczyk, M. H. , I. L. Feld, and 
E. W. Collins. Dewatering Florida Phos- 
phate Pebble Rock Slime by Freezing Tech- 
niques. BuMines RI 7520, 1971, 20 pp. 

39. Bromwell, L. G. Dewatering and 
Stabilization of Waste Clays, Slimes and 
Sludges. Pres. at Soil and Site Improve- 
ment Conf., Univ. CA, Berkeley, CA, June 
21-25, 1976. FL Inst. Phosphate Res., 
Bartow, FL, 1976, 42 pp. 

40. Hadzeriga, P. Treatment of Phos- 
phate Rock Slimes by Freezing. U.S. Pat. 
3,681,931, Aug. 8, 1972. 

41. Florida Institute of Phosphate Re- 
search (Bartow, FL) . Thermal Process 
for Rapid Dewatering of Phosphatic Clay 
Waste. Proj. Sum., Proposal 82-02-021, 
1982, 1 p. 

42. Hale, A. M. New Reclamation and 
Restoration Trends in Florida Phosphate 
Mines. Min. Eng. , (N.Y.), v. 34, No. 2, 
1982, pp. 172-176. 

43. Lawver, J. E. IMC-AGRICO-MOBIL 
Slime Consolidation and Land Reclamation 
Study. Agrico Chem. Div., Int. Miner, 
and Chem. Corp., Bartow, FL, Progress 
Rep. 6, Feb. 19, 1982, 141 pp. 



44. Florida Phosphate Council. Posi- 
tion Statement: Phosphate Mining Waste 
Disposal and Reclamation, FL Inst. Phos- 
phate Res., Bartow, FL, Apr. 1982, 89 pp. 

45. Brandt, L. W, Dewatering Florida 
Phosphatic Clay Wastes With Moving 
Screens. BuMines RI 8529, 1981, 16 pp. 

46. Bromwell, L. G. , and T. P. Oxford. 
Waste Clay Dewatering and Disposal. Pa- 
per in Proc. ASCE Specialty Conf. on Geo- 
technical Practice for Disposal of Solid 
Waste Materials, Ann Arbor, MI, 1977. 
ASCE, 1977, 19 pp. 

47. Martin, R. T., L. G. Bromwell, and 
J. H. Sholine. Field Tests of Phosphatic 
Clay Dewatering. Paper in Proc. ASCE 
Specialty Conf. on Geotechnical Practice 
for Disposal of Solid Waste Materials, 
Ann Arbor, MI, 1977. ASCE, 1977, 11 pp. 

48. ANDCO, Inc. (Buffalo, NY). Dewa- 
tering of Phosphate Slimes and Tailings 
by Andco Process. Oct., 1973, 37 pp. 

49. McLendon, J. T. Trip Report: On- 
site Inspection of ANDCO' s Pilot Plant 
Test for Dewatering Phosphate Slimes at 
USS Agri-Chem. , Ft. Meade, FL. Sept. 19, 
1973, 3 pp. Available upon request from 
Tuscaloosa Res. Cent., Bureau of Mines, 
Tuscaloosa, AL. 

50. Bromwell, L. G. Progress Report: 
Florida Phosphatic Clays Research Proj- 
ect. FL Inst, Phosphate Res., Bartow, 
FL, Apr. -June 1974, 26 pp. 

51. Whitney, E. D. Field Testing of 
Seepage Techniques for Dewatering the 
Phosphate Slimes. Cent. Res. Min. and 
Miner. Resour. , Univ. FL, Gainesville, 
FL, July 14, 1975, 6 pp. 

52. Moudgil, B. M. , T. P, Oxford, 
E. D, Whitney, and G. Y, Onoda. Field 
Test of a Seepage Technique for Dewater- 
ing Waste Phosphatic Clays, Pres. at 
103d Annu. Meeting, AIME, Dallas, TX, 
Feb. 24-28, 1974. TMS-AIME preprint A77- 
87, 14 pp. 



29 



53. Moudgil, B. M. , T. P. Oxford, and 
G, Y. Onoda. Field Test of a Seepage 
Technique for Dewatering Waste Phosphatic 
Clays. Min. Eng. , (N.Y.), v. 34, No. 3, 
1982, pp. 297-300. 

54. Bromwell, L. G. Progress Report: 
Florida Phosphatic Clays Research Proj- 
ect. FL Inst. Phosphate Res., Bartow, 
FL, Jan. -June 1976, pp. III-l to III-2. 



V. 2. Mineralogy of Phosphatic Clays. 
FL Inst. Phosphate Res., Bartow, FL, No. 
82-002-004, Dec. 1982, 109 pp. 

61. Ardaman and Associates. Evalua- 
tion of Phosphatic Clay Disposal and Rec- 
lamation Methods, V. 3. Sedimentation 
Behavior of Phosphatic Clays. FL Inst. 
Phosphate Res., Bartow, FL, No. 82-002- 
005, Dec. 1982, 255 pp. 



55. M. S. French, Inc. (Holmes Beach, 
FL). ENVIRO-CLEAR, General Catalog EC- 
76. 1976, 35 pp. 

56. Barreiro, L. J. , R. D. Austin, and 
A. P. Kouloheris. Compaction of Slimes 
and Sand Tailings by the ENVIRO-CLEAR 
Thickener, Seminar of the Phosphatic 
Clays Project. FL Inst. Phosphate Res., 
Bartow, FL, Jan. 26, 1977, 15 pp. 

57. Raden, D. J. Dewatering Phosphate 
Clay Waste Using the ENVIRO-CLEAR Thick- 
ener. Paper in Proc. Consolidation and 
Dewatering of Fine Particles Conf,, Tus- 
caloosa, AL, Aug, 10-12, 1982, Univ. AL, 
1982, pp, 205-224, 

58. Florida Institute of Phosphate Re- 
search (Bartow, FL) , Field Evaluation of 
Sand-Clay Mix Reclamation, Proj , Sum, , 
Proposal 80-03-006, 1980, 1 p, 

59. Ardaman and Associates. Evalua- 
tion of Phosphatic Clay Disposal and 
Reclamation Methods, V. 1, Index Prop- 
erties of Phosphatic Clays. FL Inst. 
Phosphate Res., Bartow, FL, No. 82-002- 
003, Dec. 1982, 100 pp. 



62. Timberlake, R. C. Building Land 
With Phosphate Wastes. Pres. at Florida 
Section, AIME, Lakeland, FL, Nov. 3, 
1969. FL Inst. Phosphate Res., Bartow, 
FL, 1969, 15 pp. 

63. Gardinier, Inc. (Tampa, FL). 
Clarif lux-Superf locculation Process ; 
Method for Treatment and Disposal of 
Phosphatic Slimes. Apr. 1981, 8 pp. 

64. Alsthom Atlantique (New Orleans, 
LA). Clarif lux TPM Water Clarif ier. Un- 
dated, 4 pp. 

65. Smelley, A. G. , and I. L. Feld. 
Flocculation Dewatering of Florida Phos- 
phatic Clay Wastes. BuMines RI 8349, 
1979, 26 pp. 

66. Scheiner, B. J., A. G. Smelley, 
and D. R. Brooks. Large-Scale Dewatering 
of Phosphatic Clay Waste From Central 
Florida. BuMines RI 8611, 1982, 11 pp. 

67. Smelley, A. G. , and B. J. Schei- 
ner. Synergism in Polyethylene Oxide De- 
watering of Phosphatic Clay Waste. Bu- 
Mines RI 8436, 1980, 18 pp. 



60. 



Evaluation of Phosphatic 



Clay Disposal and Reclamation Methods. 



IN T.-BU.OF MINES,PGH.,PA. 27 56 5 



H3 



-85 




■^' 



















r^' 






^m^r^ 



^o. 



^^V.- "I^^ '""" f 



^^^^^^K 



J^* AT "^ 



vV^^ 



i~ .r 



.^. aV -^ •.^l^.* /^ ^. 







^- ■'>. 




•^o^ 
















'•-» "^n 






'^o 








-^^^^ 















^^0^ 



<Ci°^ 



».*• 






\/ .•^\ %^^ '^' \./ .•^"t %.«* «'• \/ .•^'- %^^^ 

V- ^^% '^s /% •«^-- *■*'% '-^- /\ ".^••" **'% \W." J'% 



c V 








N 




^> 




^0^ 




'bV" 




>o^ 



.^°^ 




% ^<^ ^^1^^^* ""^o C-?^"" .'^W/V,'« «.. A^ 



^--^ 'J?^ 



4 o * 





















' . . « * .G 








'^<^^^ 





^O, aV cO""" ^ 0^ 



iP-T*. 



-'^°,^ 




/,-5a£i-.\. ./..i.;^.% ..**\.iSi'.X /..i;^- -""o /t-^>\ 











^oyr" 




>o^ 





^ " ■• a'9^ *^ ' O N ., 







<^ aO o!,*"' 









'CKMAN 

BINDERY INC. 




^ 



^^ DEC 84 

S*' INDIANA 46962 









'^o^ 

^<^^ 








^°^ 






•^ A- 



•^ "rS* *■ S?^llill^2' " c,'' ~\r> o "V//^(?\\sy " A'>"'> ■• "::^illlllllH^5' o /-"J^'rt oV/Z/^^WV* n^*^^ 





A^' -^^ 






H,; rjyihii 






002 955 830 9 



'mi 






Kill 



13 



liP ' 




